I think from analyzing the convection aspect of the problem you're right.

But in this problem, there's conduction, convection, and radiation. And, at these temperatures, the heat transfer coefficients for each will be in that order: conduction, convection, and radiation. So, the simple analysis (not always right) is that you get the biggest improvement when you help the biggest contributor. In this case, that is the conduction from device to heatsink. If we improve that the most, then that is the best solution.

__________________
The whole problem with the world is that fools and fanatics are always so certain of themselves, and wiser people so full of doubts. Bertrand Russell

But in this problem, there's conduction, convection, and radiation. And, at these temperatures, the heat transfer coefficients for each will be in that order: conduction, convection, and radiation. So, the simple analysis (not always right) is that you get the biggest improvement when you help the biggest contributor. In this case, that is the conduction from device to heatsink. If we improve that the most, then that is the best solution.

Hi leadbelly,

the conduction of heat from the device through the insulation pad into the heatsink and from there into every part of the sink is one aspect. Without convection the conduction could be as good as it wants - it wouldn`t help much to cool the devices (assumed taking aside radiation effect).
Imagine a transistor/insulation/heatsink arrangement with a thermal conductivity of infinity (of course not possible in the real world) but with zero thermal conductivity from heatsink to air the heatsinks temperature would saturate on the transistor temperature after a while - but it would not cool.

Therefore I would assume that the position of mounting the transistors does not negatively effect conductivity (at least not for steady state power dissipation) but maybe could help convection.

So the abilty of a heatsink to actually cool the devices not only depends on conductivity but it is at least as important how good the heat can be transfered to air and in turn this air away from the heatsink, respectively to bring as much amount as possible of "fresh"-air in contact with the heatsink surface - and this is what convection does.
We all know forced convection (fans) can increase the max. power dissipation ability by a very large extend and it would be great to find a way to increase convection (even only a slight increase might bring a big improvement in the max. dissipation) by mechanical arrangement of heatsinks or transistor mounting only, without using fans (I´m not a fan of fans ).

__________________www.audiosector.com
“Do something really well. See how much time it takes. It might be a product, a work of art, who knows? Then give it away cheaply, just because you feel that it should not cost so much, even if it took a lot of time and expensive materials to make it.” - JC

As You can see, I live in Hungary, here we have only one company which makes heatsinks. I know some guys from there, and they gave me some documents about the usage of the heatsinks.
There are some interesting facts.
If You use only one transistor, the best placing is the middle of the heatsink.
If You use two of them, the placing is more interesting, from the top of the heastsink:
Top - 15% - first tr. - 70% - second tr. -15% - Bottom (Ihope it's clear)
For three devices it's more complicated. If anybody interesting i will scan it, and put up to my homepage.

This is the way I positioned the transistors for my AlephX.
It's test running for days now, and they don't get too hot.
I used Sil-Pad2000 from Bergquist, which is fairly expensive (€ 1.85 each) but has good thermal ratings.
It will look a bit like the Peter_Daniel_prototype_setup_and_wiring.
Does the company also have a valid explanation why to put them like they explain? Could you check that? In building Aleph's the wiring would become much longer then I prefer. That’s why I made a 'group' on the top part of the sink.